Modification of Gold Surfaces with Water-Soluble (Co) polymers

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Modification of Gold Surfaces with Water-Soluble (Co)polymers Prepared via Aqueous Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization† Brent S. Sumerlin,‡ Andrew B. Lowe,§ Paul A. Stroud,‡ Ping Zhang,‡ Marek W. Urban,*,‡ and Charles L. McCormick*,‡,§ Department of Polymer Science and Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-0076 Received March 17, 2003. In Final Form: May 8, 2003 Reversible addition-fragmentation chain transfer (RAFT) is a versatile, controlled free radical polymerization technique that operates via a degenerative transfer mechanism in which a thiocarbonylthio compound acts as a chain transfer agent. The subsequent reduction of the dithioester end groups to thiols allows the preparation of (co)polymer-modified gold surfaces. Herein we report the immobilization of poly(sodium 4-styrenesulfonate), poly((ar-vinylbenzyl) trimethylammonium chloride), poly(N,N-dimethylacrylamide), and poly(3-[2-(N-methylacrylamido)-ethyldimethyl ammonio]propane sulfonate-b-N,Ndimethylacrylamide) onto gold films. The presence of the immobilized (co)polymers was confirmed by atomic force microscopy, attenuated total reflectance Fourier transform infrared spectroscopy, and surface contact angle measurements. The gold film modified with the block copolymer demonstrated stimuliresponsive behavior as evidenced by its water contact angle being similar to that of poly(N,Ndimethylacrylamide) even though the block based on 3-[2-(N-methylacrylamido)-ethyldimethyl ammonio] propane sulfonate was expected to be exposed to the aqueous environment.

Introduction 1,2

Due to numerous potential applications in electronics, catalysis,3,4 and biotechnology,5-7 self-assembled monolayers (SAMs) of organic molecules containing thiol or disulfide moieties have become the focus of numerous studies.8 In a recent example reported by Lahann et al., gold was functionalized with carboxylated alkanethiols to yield a modified surface with tunable hydrophilicity in response to an applied electric potential.9 Most research to date has focused on the preparation of SAMs by the adsorption of low molecular weight molecules onto gold, though other transition metals have also been modified. There have been relatively few reports concerning surface modification with well-defined (co)polymers by the “grafting to” approach, and most have involved the modification of transition metal colloids.10-14 The adsorption of polymers onto gold films has received considerably less attention.15-17 * To whom correspondence should be addressed. Fax: (601) 2665635. E-mail: [email protected], [email protected]. † Number 95 in a series entitled “Water-Soluble Polymers”. ‡ Department of Polymer Science. § Department of Chemistry and Biochemistry. (1) Osifchin, R. G.; Andres, R. P.; Henderson, J. I.; Kubiak, C. P.; Dominey, R. N. Nanotechnology 1996, 7, 412. (2) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. Appl. Phys. 1997, 82, 696. (3) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364. (4) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217. (5) de la Fuente, J.; Barrientos, A. G.; Rojas, T. C.; Canada, J.; Fernandez, A.; Penades, S. Angew. Chem., Int. Ed. 2001, 40, 2257. (6) Cao, Y.-W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (7) Li, Z.; Jin, R.; Mirkin, C. A.; Letsinger, R. L. Nucleic Acids Res. 2002, 30, 1558. (8) Ulman, A. Chem. Rev. 1996, 96, 1533. (9) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 321. (10) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696. (11) Teranishi, T.; Hosoe, M.; Miyake, M. Adv. Mater. 1997, 9, 65. (12) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596.

Recently, we reported a novel route for the facile preparation of (co)polymer-stabilized transition metal nanoparticles.18 Significantly, the (co)polymers employed as stabilizers were synthesized by reversible additionfragmentation chain transfer (RAFT) polymerization in aqueous media. RAFT is a versatile, controlled free radical polymerization technique that operates via a degenerative transfer mechanism in which a thiocarbonylthio compound acts as a chain transfer agent (CTA).19 By virtue of the mechanism, (co)polymers prepared by this technique bear dithioester end groups. The reduction of these dithioesters to thiols in the presence of a suitable transition metal complex leads to the formation of (co)polymer-stabilized metal nanoparticles. Herein, we report the extension of this work to the modification of gold films with spectroscopic evidence confirming the presence of the immobilized species. The ability to immobilize well-defined, stimuliresponsive (co)polymers with R,ω-telechelic functionality to planar supports has potential applications in the rapidly developing field of bioarrays. Specifically, it has become increasingly important to find adequate spacer molecules that separate bimolecular ligands from the surface to which they are attached in order to preserve their functionality.20 Employing hydrophilic (co)polymers in such applications may be advantageous by reducing the extent of nonspecific, hydrophobic adsorption between the spacer molecule and the bimolecular ligand. (13) Jordan, R.; West, N.; Ulman, A.; Chou, Y. M.; Nyuken, O. Macromolecules 2001, 34, 1606. (14) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruhm, A.; Lenox, R. B. J. Am. Chem. Soc. 2001, 123, 10411. (15) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Haussling, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (16) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364. (17) El Sayed, A. M. J. Appl. Polym. Sci. 2002, 86, 1248. (18) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562. (19) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (20) Kasemo, B. Surf. Sci. 2002, 500, 656.

10.1021/la034459t CCC: $25.00 © 2003 American Chemical Society Published on Web 06/07/2003

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Experimental Section General Polymerization Conditions. All reagents were purchased from Aldrich Chemical Co. (Milwaukee, WI) at the highest purity available. Each polymerization was conducted under a nitrogen atmosphere in a 25 mL round-bottomed flask equipped with a magnetic stir bar and sealed with a rubber septum. 4,4′-Azobis(4-cyanopentanoic acid) (Wako Chemical, USA) and 4-cyanopentanoic acid dithiobenzoate21 were employed as the initiator and RAFT CTA, respectively. The [CTA]/[initiator] remained constant at 5:1 (mol basis). The monomer concentration was 2.0 M. The (co)polymers were purified by dialysis against deionized water and isolated by lyophilization. Additional systemspecific details follow. Homopolymers of sodium 4-styrenesulfonate (NaPSS) and (arvinylbenzyl) trimethylammonium chloride (VBTAC) were synthesized in water (pH 7.0) at 70 °C (PNaPSS: Mn ) 19 800, PDI ) 1.12; PVBTAC: Mn ) 10 500, PDI ) 1.06).21 The homopolymerization of N,N-dimethylacrylamide (DMA) was conducted in water (pH 7.5) at 70 °C (Mn ) 29 100, PDI ) 1.18).22 An AB diblock copolymer of 3-[2-(N-methylacrylamido)-ethyldimethyl ammonio] propane sulfonate (MAEDAPS) and DMA was prepared by first synthesizing PMAEDAPS in 0.5 M NaBr. The resulting homopolymer was then used as a macro-CTA, with 4,4′-azobis(4-cyanopentanoic acid) as the initiator, allowing the synthesis of a block copolymer in 0.5 M NaBr. (P(MAEDAPSb-DMA); Mn ) 58 700, PDI ) 1.19, MAEDAPS/DMA ) 35:65).23 Immobilization of RAFT-Prepared (Co)polymers onto Gold Films. Gold-coated glass slides (1 cm × 1 cm × 1000 Å) were obtained from EMF Corp. (Ithaca, NY). Immediately prior to use, the slides were immersed for 2 min in 3:1 concentrated H2SO4/30% H2O2 (“piranha” solution; Caution: piranha solution reacts violently with organic materials) at 80 °C, rinsed with deionized water, and dried under a nitrogen atmosphere. The surface modification reactions involved dropwise addition of aqueous NaBH4 (0.5 mL, 1.0 M) to a solution of dithioester endcapped polymer in deionized water (5.0 mL, 0.1 mM) in the presence of the gold-coated slides. The thiol-terminated polymers were left to react with the gold-coated slides for 48 h. The slides were removed from the supernatant, rinsed by constant agitation in deionized water for 48 h, and dried under a nitrogen atmosphere. Characterization. Tapping mode atomic force microscopy (AFM) images were collected using a Digital Instruments Dimension 3000 scanning probe microscope. Each slide was examined at a minimum of three different locations on the sample surface. Surface hydrophobicity was examined by performing water contact angle measurements with a First Ten Angstroms FTÅ 200 dynamic contact angle analyzer. Three sets of contact angle measurements were collected using a 10 µL drop size of deionized, distilled water. Between measurements, samples were dried in an oven at 55 °C for 10 min. Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were obtained with a Digilab FTS-6000 FT-IR single-beam spectrometer set at a 4 cm-1 resolution. A 45° face angle Ge crystal with 50 × 20 × 3 mm dimensions was used. This configuration allows the analysis of the film-air interface from monolayer levels to approximately 0.2 µm from the surface. Each spectrum represents 5000 coadded scans ratioed to 5000 coadded reference scans that were collected using an empty ATR cell. All spectra were corrected for spectral distortions and optical effects using Q-ATR software.24 Molecular weights and polydispersities were determined by aqueous size exclusion chromatography.21-23 (21) Mitsukami, Y.; Donovan, M. S.; Lowe, A. B.; McCormick, C. L. Macromolecules 2001, 34, 2248. (22) Donovan, M. S.; Sanford, T. A.; Lowe, A. B.; Sumerlin, B. S.; Mitsukami, Y.; McCormick, C. L. Macromolecules 2002, 35, 4570. (23) Donovan, M. S.; Sumerlin, B. S.; Lowe, A. B.; McCormick, C. L. Macromolecules 2002, 35, 8663. (24) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers; American Chemical Society and Oxford University Press: Washington, D.C., 1996.

Letters Scheme 1. (A) Basic Mechanism Describing the Equilibrium between Dormant and Active Chains in the RAFT Process and (B) the Structures of the (Co)polymers Employed for the Modification of Gold Surfaces

Scheme 2. Basic Mechanism Describing the In Situ Reduction and Immobilization of a RAFT-Prepared (Co)polymer on a Gold Surface

Results and Discussion RAFT is a controlled free radical polymerization technique that allows the synthesis of well-defined, controlledstructure (co)polymers through an equilibrium between dormant and active chains (Scheme 1A). Due to the versatility of the process, functional (co)polymers can be prepared directly in aqueous media. As representative examples, we chose a range of (co)polymers composed of anionic, cationic, zwitterionic (betaine), and neutral species (Scheme 1B). The addition of aqueous NaBH4 to a solution of each (co)polymer in the presence of a gold film resulted in the in situ reduction of the thiocarbonylthio end groups to thiols and their subsequent surface adsorption (Scheme 2). Chemisorption of the thiol end groups is evidenced by the fact that the (co)polymers remained immobilized after thorough rinsing. Furthermore, no adsorption has been observed for polymers with similar

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Figure 1. ATR FT-IR spectra with tentative band assignments for unmodified gold (A), gold modified with PVBTAC (B), PVBTACmodified gold minus unmodified gold (C), and bulk PVBTAC (D), over four specific wavenumber regions. ATR FT-IR: 3389 (OH stretch (str)), 3026 (aromatic CH str or C-N(CH3)3+ str), 1512 (aromatic CC str), 1479, 1427 (aromatic CH str), 1489, 1418, 977 (C-N(CH3)3+ str), 1222 (CH or CN bend), 977, 891, 859, 831 cm-1 (aromatic CH wag).

molecular weights and polydispersities that did not contain dithioester end groups.18 Characterization by ATR FT-IR spectroscopy and AFM of unmodified and (co)polymer-modified gold films confirmed the presence of the immobilized monolayers. Figure 1 shows ATR FT-IR spectra for four different wavenumber regions for the gold film modified with PVBTAC. Despite being exposed to the piranha solution for 2 min at 80 °C, the presence of adventitious organic material was apparent in the ATR FT-IR spectrum for the unmodified gold surface (Figure 1A). The adsorption of such material occurs under ambient laboratory conditions in a matter of minutes due to the high free energy of the gold surface25,26 but has been shown to not significantly interfere with the binding of thiol-containing compounds.27 The subtraction of the spectrum for the unmodified gold from the spectrum obtained for PVBTAC-modified gold resulted in a spectrum nearly identical to that obtained for the bulk PVBTAC sample used in the immobilization procedure (Figure 1C,D). Additional evidence for the successful immobilization of the (co)polymers can be seen in the AFM images contained in Figure 2. The contrast in the phase images (25) Loeser, E. H.; Harkins, W. D.; Twiss, S. B. J. Phys. Chem. 1953, 57, 251. (26) Gaines, G. L., Jr. Colloid Interface Sci. 1981, 79, 295. (27) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

Figure 2. Phase images of gold films modified with PVBTAC (A), NaPSS (B), PDMA (C), and P(MAEDAPS-b-DMA) (D) obtained by tapping mode AFM (Z-range ) 120°).

denotes the areas of immobilized (co)polymer. The films modified with PVBTAC and NaPSS demonstrated slightly

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Letters

Table 1. Water Contact Angle Measurement Results Obtained for Gold Films Modified with Various (Co)polymersa water contact angle (deg) no polymer NaPSS PDMA P(MAEDAPS-b-DMA) PMAEDAPS PVBTAC

75.9 67.3 30.5 29.1 41.5 41.1

a The values represent the average of three 10 µL drops of distilled, deionized water.

higher degrees of surface coverage than those modified with PDMA and P(MAEDAPS-b-DMA). This could be attributed tentatively to the more extended nature of the polyelectrolytes during adsorption, which might allow the dithioester moieties to be more easily accessible to the surface. Table 1 contains the results from water contact angle measurements. All of the films demonstrated reduced contact angles as compared to those of the unmodified gold, thus indicating a surface transition to a more hydrophilic state. The contact angle obtained for the unmodified gold containing adventitious, nonpolar material is similar to that reported by other groups.27,28 The reduction in contact angle following the polymer immobilization procedures would be expected since the (co)polymers employed in this study are extremely hydrophilic. Interestingly, the contact angle for the P(MAEDAPSb-DMA) sample (29.1°) was nearly identical to that of the PDMA-modified gold (30.5°) and similar to the contact angle reported by Baum et al. for PDMA brushes grafted from a silicate surface (33°).29 This result was striking because the dithioester resides at the terminus of the DMA block, and following attachment, the outer block is expected to be MAEDAPS. However, due to the similarity of the contact angles observed for the block copolymer and PDMA samples, we hypothesized the DMA block was exposed. To gain further insight, the contact angle was determined for a gold film modified with MAEDAPS ho(28) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (29) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610.

mopolymer. PMAEDAPS is derived from a sulfobetaine monomer that contains both positive and negative charges on the same repeat unit. As a result of the electrostatic interactions between the opposite charges, polybetaines are not soluble in deionized water but are soluble in aqueous salt solutions. The water contact angle for the gold modified with PMAEDAPS was determined to be 41.5°, indicating the surface was more hydrophobic than the gold modified with the block copolymer. This result, coupled with the similarity observed between the contact angles of the PDMA and the P(MAEDAPS-b-DMA) samples, suggests the relatively hydrophobic MAEDAPS block causes the block copolymer to adopt a conformation such that the more hydrophilic DMA block is exposed to the aqueous environment. Rearrangement of the blocks most likely occurred when the sample was treated with deionized water during the rinsing step that immediately followed the immobilization procedure. This solventselective behavior has been observed by others for diblock copolymers attached to solid surfaces.29,30 Conclusions RAFT polymerization and the subsequent reduction of the dithioester end groups to thiols allow the preparation of (co)polymer-modified gold surfaces. Due to the versatility of the RAFT process, a wide range of (co)polymers with controlled molecular weights and architectures can be immobilized via this facile method without the need for extensive surface purification. Because the (co)polymers investigated were prepared with 4,4′-azobis(4-cyanopentanoic acid) and 4-cyanopentanoic acid dithiobenzoate as the initiator and CTA, respectively, the free end of each surface-bound chain contains a terminal carboxyl group capable of further reaction. This provides an avenue for chemical modification that could prove useful for biochip and high-throughput screening applications. Acknowledgment. We gratefully acknowledge the financial support for this research provided by GelTex Pharmaceuticals, Incorporated, the U.S. Department of Energy, and the MRSEC program of the National Science Foundation under Award Number DMR-0213883. LA034459T (30) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407.